Telegraph City
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Telegraph City clear sky chart from 2005-11-06 to 2025-07-21.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	7% 3 1 3 2 2 2 2 4 5% 67%	123974
	Night	9% 4 1 3 2 2 3 2 4 5% 63%	124031
Transparency	Day	21% 3 7% 18% 25% 25%	123728
	Night	24% 4 9% 19% 24% 20%	123864
Seeing	Day	8% 5% 21% 35% 24% 7%	16146
	Night	10% 3 14% 41% 26% 5%	25514

By Month

Percentage Cloud Cover SW (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	16% 6% 2 5% 3 3 3 3 5% 8% 46%	3639
	11th to 20th	Day	14% 6% 3 6% 4 4 3 2 3 4 52%	4031
	21st to end	Day	16% 6% 3 6% 4 4 4 4 5% 7% 42%	4510
February	1st to 10th	Day	19% 6% 2 4 4 2 3 3 4 5% 47%	1800
	11th to 20th	Day	14% 5% 2 5% 4 3 3 4 5% 8% 46%	1931
	21st to end	Day	16% 5% 2 5% 4 4 3 4 4 6% 48%	1650
March	1st to 10th	Day	14% 8% 3 8% 4 5% 4 4 6% 7% 37%	4606
	11th to 20th	Day	16% 5% 2 4 2 3 3 4 6% 7% 50%	5000
	21st to end	Day	13% 7% 2 5% 4 4 4 4 6% 8% 43%	5739
April	1st to 10th	Day	12% 5% 2 3 3 3 4 4 5% 7% 52%	2400
	11th to 20th	Day	9% 4 1 4 2 2 4 3 5% 6% 60%	2369
	21st to end	Day	7% 4 1 3 2 2 3 3 3 6% 65%	2400
May	1st to 10th	Day	4 3 1 2 1 1 1 1 3 4 77%	5456
	11th to 20th	Day	2 2 2 1 1 2 2 3 4 81%	6202
	21st to end	Day	2 1 1 1 1 1 2 2 3 4 84%	6902
June	1st to 10th	Day	3 2 1 3 2 2 2 2 3 6% 73%	2800
	11th to 20th	Day	1 1 1 1 1 2 1 3 5% 85%	2800
	21st to end	Day	2 1 1 1 1 1 1 3 2 86%	2800
July	1st to 10th	Day	1 1 1 1 94%	4560
	11th to 20th	Day	1 1 2 1 1 2 2 2 4 84%	4543
	21st to end	Day	1 1 1 1 1 1 2 2 90%	4456
August	1st to 10th	Day	2 1 1 2 1 2 1 1 3 3 83%	4068
	11th to 20th	Day	1 1 1 1 1 1 2 3 90%	3724
	21st to end	Day	1 1 1 1 1 1 1 2 2 89%	4180
September	1st to 10th	Day	1 1 1 1 1 1 1 1 3 90%	2280
	11th to 20th	Day	2 1 1 2 1 2 2 2 3 4 79%	2280
	21st to end	Day	2 1 1 2 1 1 2 2 2 4 82%	2067
October	1st to 10th	Day	2 1 1 2 2 2 2 2 3 5% 78%	3180
	11th to 20th	Day	3 1 1 2 1 2 2 1 3 3 80%	3161
	21st to end	Day	5% 2 1 3 2 3 3 3 5% 6% 68%	3448
November	1st to 10th	Day	7% 3 2 4 4 4 3 3 6% 7% 57%	1802
	11th to 20th	Day	9% 5% 2 4 4 3 3 3 5% 6% 56%	1800
	21st to end	Day	12% 4 2 4 3 4 4 3 5% 8% 52%	1738
December	1st to 10th	Day	8% 4 1 4 4 4 4 3 4 8% 57%	2620
	11th to 20th	Day	24% 8% 3 6% 4 3 3 3 5% 7% 34%	3028
	21st to end	Day	20% 8% 3 7% 5% 5% 5% 5% 8% 9% 26%	4004
January	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 6% 9% 51%	6306
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 4 5% 53%	5941
	21st to end	Night	16% 6% 2 5% 4 3 3 3 4 6% 48%	6468
February	1st to 10th	Night	18% 6% 2 5% 3 3 3 3 4 5% 51%	2600
	11th to 20th	Night	15% 5% 2 4 3 3 4 3 5% 7% 50%	2355
	21st to end	Night	17% 5% 1 3 3 3 3 3 4 6% 53%	1980
March	1st to 10th	Night	16% 9% 2 6% 4 4 4 4 5% 6% 39%	5271
	11th to 20th	Night	17% 7% 2 5% 3 3 2 2 3 5% 51%	5000
	21st to end	Night	18% 7% 2 4 3 3 4 3 5% 7% 44%	5236
April	1st to 10th	Night	11% 4 2 3 3 3 3 4 5% 7% 54%	2000
	11th to 20th	Night	8% 4 2 3 3 3 3 3 4 7% 60%	1979
	21st to end	Night	5% 4 1 2 3 2 3 3 4 7% 66%	2000
May	1st to 10th	Night	2 2 1 3 2 1 2 2 3 5% 77%	4542
	11th to 20th	Night	2 1 2 2 1 2 2 3 5% 80%	3690
	21st to end	Night	1 1 1 1 1 1 1 2 4 85%	3972
June	1st to 10th	Night	3 2 1 2 2 1 2 2 3 5% 76%	1600
	11th to 20th	Night	1 1 1 1 2 1 3 4 87%	1600
	21st to end	Night	1 1 1 2 1 1 1 1 2 3 87%	1600
July	1st to 10th	Night	1 1 1 1 1 95%	2620
	11th to 20th	Night	2 2 1 2 2 1 2 2 2 3 82%	2620
	21st to end	Night	1 1 1 1 1 1 1 3 3 86%	3109
August	1st to 10th	Night	2 1 1 2 1 1 2 2 3 4 81%	2870
	11th to 20th	Night	1 1 1 1 1 1 1 2 3 89%	3126
	21st to end	Night	3 1 1 1 1 1 1 1 1 90%	3498
September	1st to 10th	Night	1 1 1 1 1 1 1 1 2 90%	1900
	11th to 20th	Night	2 2 1 2 1 2 1 1 3 4 82%	1900
	21st to end	Night	2 1 1 1 1 1 1 1 2 3 85%	2107
October	1st to 10th	Night	3 1 1 2 1 2 2 2 3 3 80%	3800
	11th to 20th	Night	3 1 1 2 1 2 2 1 3 4 81%	3777
	21st to end	Night	6% 2 1 3 2 2 3 2 4 5% 70%	4121
November	1st to 10th	Night	10% 4 1 4 3 1 2 3 4 7% 62%	2495
	11th to 20th	Night	8% 4 2 3 3 2 3 3 5% 7% 59%	2600
	21st to end	Night	12% 4 2 3 3 3 3 3 4 6% 57%	2660
December	1st to 10th	Night	9% 3 1 3 3 3 4 3 5% 6% 61%	4560
	11th to 20th	Night	22% 7% 2 6% 4 4 4 4 6% 9% 33%	5176
	21st to end	Night	19% 8% 3 8% 5% 5% 5% 4 7% 7% 29%	6952

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	40% 6% 19% 22% 10% 3	3639
	11th to 20th	Day	37% 6% 6% 14% 26% 10%	4031
	21st to end	Day	42% 6% 9% 14% 27% 3	4510
February	1st to 10th	Day	40% 6% 10% 23% 15% 7%	1800
	11th to 20th	Day	36% 5% 12% 22% 21% 6%	1941
	21st to end	Day	37% 5% 11% 24% 20% 4	1650
March	1st to 10th	Day	42% 7% 10% 21% 18% 1	4606
	11th to 20th	Day	31% 6% 13% 29% 13% 7%	5000
	21st to end	Day	36% 6% 14% 30% 13% 2	5739
April	1st to 10th	Day	30% 6% 12% 27% 18% 8%	2400
	11th to 20th	Day	24% 5% 13% 27% 23% 8%	2372
	21st to end	Day	20% 3 12% 29% 29% 7%	2400
May	1st to 10th	Day	13% 2 5% 27% 34% 19%	5456
	11th to 20th	Day	9% 2 5% 28% 41% 15%	6205
	21st to end	Day	7% 2 6% 21% 47% 16%	6904
June	1st to 10th	Day	14% 3 9% 25% 32% 18%	2800
	11th to 20th	Day	6% 1 6% 19% 33% 35%	2800
	21st to end	Day	8% 1 2 9% 31% 50%	2800
July	1st to 10th	Day	2 1 1 4 13% 80%	4491
	11th to 20th	Day	7% 2 2 5% 13% 71%	4468
	21st to end	Day	5% 1 1 3 21% 69%	4316
August	1st to 10th	Day	9% 1 2 5% 19% 65%	4053
	11th to 20th	Day	4 1 1 5% 27% 62%	3760
	21st to end	Day	6% 1 3 6% 32% 53%	4179
September	1st to 10th	Day	5% 1 2 6% 26% 60%	2280
	11th to 20th	Day	10% 2 4 18% 39% 26%	2280
	21st to end	Day	9% 1 4 17% 38% 32%	2067
October	1st to 10th	Day	10% 2 4 11% 22% 51%	3180
	11th to 20th	Day	11% 2 5% 23% 37% 22%	3161
	21st to end	Day	18% 3 5% 20% 40% 15%	3448
November	1st to 10th	Day	26% 3 9% 27% 24% 10%	1802
	11th to 20th	Day	29% 5% 12% 27% 23% 4	1800
	21st to end	Day	31% 5% 11% 23% 22% 7%	1738
December	1st to 10th	Day	28% 5% 13% 29% 21% 4	2620
	11th to 20th	Day	51% 6% 10% 18% 13% 2	3028
	21st to end	Day	51% 11% 17% 12% 7% 1	4004
January	1st to 10th	Night	31% 7% 21% 27% 10% 4	6306
	11th to 20th	Night	34% 5% 10% 17% 26% 8%	5941
	21st to end	Night	38% 4 9% 16% 26% 7%	6468
February	1st to 10th	Night	36% 3 9% 23% 19% 8%	2600
	11th to 20th	Night	32% 5% 10% 23% 23% 7%	2356
	21st to end	Night	34% 3 10% 28% 19% 5%	1980
March	1st to 10th	Night	42% 7% 10% 17% 20% 4	5271
	11th to 20th	Night	36% 4 11% 25% 14% 9%	5000
	21st to end	Night	38% 6% 13% 26% 13% 3	5236
April	1st to 10th	Night	28% 5% 14% 25% 22% 7%	2000
	11th to 20th	Night	24% 5% 13% 28% 20% 10%	1979
	21st to end	Night	19% 4 12% 29% 29% 7%	2000
May	1st to 10th	Night	11% 3 10% 26% 33% 18%	4539
	11th to 20th	Night	10% 2 5% 29% 40% 14%	3690
	21st to end	Night	7% 2 6% 27% 49% 10%	3972
June	1st to 10th	Night	12% 3 10% 24% 35% 16%	1600
	11th to 20th	Night	4 2 6% 18% 36% 35%	1600
	21st to end	Night	6% 1 2 12% 30% 49%	1600
July	1st to 10th	Night	2 1 4 12% 80%	2580
	11th to 20th	Night	9% 2 2 4 13% 70%	2572
	21st to end	Night	6% 1 2 3 22% 65%	3011
August	1st to 10th	Night	9% 1 2 7% 19% 63%	2861
	11th to 20th	Night	4 1 1 5% 28% 59%	3156
	21st to end	Night	6% 1 1 8% 30% 54%	3498
September	1st to 10th	Night	5% 3 5% 27% 59%	1900
	11th to 20th	Night	9% 1 4 19% 39% 28%	1900
	21st to end	Night	8% 1 3 16% 38% 34%	2107
October	1st to 10th	Night	11% 2 4 11% 18% 55%	3800
	11th to 20th	Night	10% 2 5% 25% 34% 24%	3777
	21st to end	Night	17% 2 5% 18% 36% 22%	4121
November	1st to 10th	Night	22% 3 9% 25% 24% 17%	2495
	11th to 20th	Night	24% 4 10% 26% 27% 9%	2600
	21st to end	Night	28% 4 9% 22% 28% 8%	2660
December	1st to 10th	Night	25% 4 10% 29% 26% 7%	4560
	11th to 20th	Night	47% 6% 11% 19% 14% 3	5176
	21st to end	Night	51% 9% 14% 16% 6% 3	6952

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	29% 10% 20% 24% 13% 5%	200
	11th to 20th	Day	17% 6% 20% 30% 20% 8%	370
	21st to end	Day	18% 4 19% 29% 23% 8%	440
February	1st to 10th	Day	23% 7% 19% 24% 20% 8%	400
	11th to 20th	Day	16% 8% 28% 26% 16% 6%	486
	21st to end	Day	15% 7% 22% 36% 17% 2	414
March	1st to 10th	Day	19% 8% 23% 33% 14% 3	500
	11th to 20th	Day	22% 8% 22% 26% 16% 6%	500
	21st to end	Day	17% 9% 25% 29% 17% 3	550
April	1st to 10th	Day	15% 9% 30% 24% 17% 6%	500
	11th to 20th	Day	11% 10% 29% 29% 15% 5%	492
	21st to end	Day	6% 9% 26% 32% 19% 8%	500
May	1st to 10th	Day	5% 12% 27% 34% 17% 5%	503
	11th to 20th	Day	5% 9% 27% 33% 22% 4	528
	21st to end	Day	5% 7% 22% 37% 25% 5%	573
June	1st to 10th	Day	4 7% 21% 39% 25% 5%	530
	11th to 20th	Day	1 6% 29% 41% 20% 2	530
	21st to end	Day	2 2 21% 42% 27% 6%	530
July	1st to 10th	Day	1 18% 46% 31% 4	526
	11th to 20th	Day	1 1 11% 46% 37% 5%	524
	21st to end	Day	1 1 10% 48% 34% 7%	532
August	1st to 10th	Day	3 1 14% 45% 29% 7%	487
	11th to 20th	Day	2 1 21% 43% 26% 7%	477
	21st to end	Day	1 1 18% 43% 32% 6%	539
September	1st to 10th	Day	2 1 14% 48% 27% 8%	490
	11th to 20th	Day	3 3 26% 37% 25% 7%	490
	21st to end	Day	3 4 15% 40% 29% 8%	490
October	1st to 10th	Day	3 7% 19% 32% 28% 11%	490
	11th to 20th	Day	6% 6% 17% 28% 32% 13%	490
	21st to end	Day	7% 4 17% 34% 27% 10%	539
November	1st to 10th	Day	8% 5% 14% 28% 29% 15%	338
	11th to 20th	Day	8% 8% 14% 35% 24% 10%	300
	21st to end	Day	12% 3 16% 31% 32% 6%	268
December	1st to 10th	Day	15% 3 17% 33% 20% 13%	200
	11th to 20th	Day	17% 7% 20% 26% 23% 9%	200
	21st to end	Day	19% 8% 23% 28% 16% 5%	220
January	1st to 10th	Night	20% 5% 15% 31% 23% 7%	800
	11th to 20th	Night	15% 3 12% 38% 23% 8%	800
	21st to end	Night	19% 4 10% 35% 26% 6%	880
February	1st to 10th	Night	19% 4 15% 35% 23% 5%	800
	11th to 20th	Night	17% 5% 16% 34% 23% 6%	784
	21st to end	Night	19% 4 14% 44% 16% 4	660
March	1st to 10th	Night	21% 4 17% 36% 19% 3	800
	11th to 20th	Night	19% 4 15% 34% 22% 6%	800
	21st to end	Night	18% 4 18% 38% 20% 2	880
April	1st to 10th	Night	14% 3 23% 36% 22% 2	800
	11th to 20th	Night	8% 5% 21% 41% 20% 5%	791
	21st to end	Night	7% 3 19% 45% 22% 4	800
May	1st to 10th	Night	5% 3 22% 44% 21% 4	795
	11th to 20th	Night	5% 3 20% 46% 24% 2	429
	21st to end	Night	3 2 21% 48% 25% 1	433
June	1st to 10th	Night	4 2 23% 45% 24% 2	400
	11th to 20th	Night	2 20% 52% 25% 2	400
	21st to end	Night	2 2 16% 52% 28% 2	400
July	1st to 10th	Night	12% 57% 30% 1	390
	11th to 20th	Night	2 1 12% 60% 25% 1	388
	21st to end	Night	1 1 10% 46% 37% 5%	624
August	1st to 10th	Night	2 11% 46% 36% 3	576
	11th to 20th	Night	1 15% 49% 30% 4	752
	21st to end	Night	2 11% 49% 34% 3	836
September	1st to 10th	Night	1 1 10% 49% 35% 5%	760
	11th to 20th	Night	3 2 14% 44% 32% 5%	760
	21st to end	Night	2 1 12% 39% 37% 8%	760
October	1st to 10th	Night	5% 2 11% 42% 32% 7%	760
	11th to 20th	Night	5% 3 12% 37% 35% 8%	760
	21st to end	Night	5% 2 12% 39% 29% 12%	836
November	1st to 10th	Night	11% 2 10% 38% 29% 10%	780
	11th to 20th	Night	11% 3 12% 40% 28% 7%	800
	21st to end	Night	14% 2 13% 37% 28% 6%	800
December	1st to 10th	Night	16% 3 11% 40% 24% 7%	800
	11th to 20th	Night	23% 2 11% 37% 23% 4	800
	21st to end	Night	21% 5% 15% 36% 19% 4	880

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